1. Field of the Invention
This invention relates generally to a semiconductor device and a method of manufacturing the same, and more specifically to a structure of a MOS transistor using two types of semiconductors as conductive layer materials, such as Si and Ge (SiGe).
2. Description of the Related Art
Using SiGe as a conductive layer material for high performance MOS transistors has been a great deal of attention.
For example, as shown in FIG. 25, SiGe is used as a gate electrode material to improve the activation efficiency of a PMOS transistor and to thereby suppress depletion. Generally, an oxide film (sidewall oxide film) of about several nm to 10 nm is formed on sidewalls of a gate electrode. Principally, the sidewall oxide film of the gate electrode is formed to eliminate damage introduced to gate edge regions of the semiconductor substrate side through mainly RIE (reactive ion etching) in the gate electrode processing. The sidewall oxide film of the gate electrode concurrently serves as a barrier layer that suppresses dopants being diffused out from the gate electrode (“out-diffusion” of dopants) during a heat treatment such as activation annealing in a later performed process.
In a MOS transistor in the generation that uses SiGe for a gate electrode, the gate electrode is required to contain carriers at a high-concentration to suppress the gate depletion. As such, the prevention of out-diffusion of dopant from the gate electrode becomes more important.
In addition, in a small-size MOS transistor, a gate-electrode sidewall oxide film is used as a spacer for providing offsets between the gate electrode and an ion-implantation region when performing ion implantation to suppress a short channel effect. Use of a SiGe film for a gate electrode is described in Jpn. Pat. Appln. KOKAI Publication No. 2002-26318 (FIG. 1, pp. 2 to 3). Meanwhile, SiGe is not a compound, but is a mixed crystal; and it is formerly represented as “Si1-xGex”.
Since the SiGe-gate sidewall oxide film is formed by oxidizing a SiGe gate electrode (SiGe), the sidewall oxide film contains SiO2 and GeO2. However, when SiO2 and GeO2 formed by oxidizing SiGe are compared with each other, GeO2 is lower than SiO2 in chemical resistance to H2O2, H2SO4, and HF, for example, and is higher in volatility than SiO2. Because of these characteristics, the sidewall oxide film is damaged by processes such as chemical and heat treatments performed after formation of the gate oxide film, and the film becomes a coarse film. As a result, the resistance to the out-diffusion of dopants from the gate electrode is significantly reduced, when the film undergoes a heat treatment such as activation annealing in a later performed process. Further, in the sidewall oxide film, thickness uniformity is also reduced, so that the function as the offset spacer between the gate electrode and the ion-implanted region is deteriorated.
As shown in FIG. 26, in the field of MOS transistors, there is a promising technique in which a SiGe film is epitaxially grown to have a SiGe channel layer to enhance the carrier mobility in the channel region. In this case, after the SiGe channel layer is formed, the surface (SiGe) of the SiGe channel layer is oxidized to form a gate oxide film. Since the gate oxide film is thus formed through oxidation of SiGe then the gate oxide film contains SiO2 and GeO2. However, as described above in connection with the sidewall oxide film of the SiGe gate electrode, when SiO2 and GeO2 formed through oxidation of SiGe are compared with each other, GeO2 is found lower to SiO2 in the chemical resistance to H2O2, H2SO4, and HF, for example. Concurrently, GeO2 is found higher than SiO2 in volatility. Accordingly, the gate oxide film thus produced is damaged during processes such as chemical and heat treatments performed after the formation of the gate oxide film. This makes the problem more prominent in that the thickness of the gate oxide film is further reduced.